A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ            NA                                              (        1        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.
EUV radiation may be produced using a plasma. A radiation source for producing EUV radiation may include an excitation beam, such as a laser, for exciting a fuel to provide the plasma, and an enclosure for containing the plasma. The plasma may be created, for example, by directing a laser beam (i.e., an excitation beam providing radiation for initiation of the plasma) at a fuel, such as droplets (i.e., droplets) of a suitable fuel material (e.g., tin, which is currently thought to be the most promising and thus likely choice of fuel for EUV radiation sources), or at a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector (sometimes referred to as a near normal incidence radiation collector), which receives the radiation from the plasma and focuses the radiation into a beam. The radiation source may include an enclosure or chamber arranged to provide a vacuum environment to support the plasma, and typically the radiation collector will be located within the enclosure. Such a radiation system is typically termed a laser produced plasma (LPP) source, when a laser is used to provide the beam of excitation radiation. In an alternative system, which may also employ the use of a laser, radiation may be generated by a plasma formed by the use of an electrical discharge—a discharge produced plasma (DPP) source.
The present application is concerned with radiation sources, and methods of generation of radiation, particularly EUV radiation for use in lithography, where the radiation, is produced from a plasma generated by excitation of a fuel by means of an excitation beam, which may typically be a laser beam, such as an infra-red laser beam. Such radiation sources include LPP radiation sources and for the sake of brevity, such a source is referred to hereinafter as an LPP radiation source, although it will be understood that the excitation beam is not necessarily limited to being a laser beam, and any other suitable excitation beam (or combination of excitation beams) may be employed. From a plasma, the radiation is typically generated at a first focal point of a radiation collector, and focused by the radiation collector to form an image at a second focal point. The second focal point is often referred to as the intermediate focus.
It is desirable to obviate or mitigate at least one problem of the prior art, whether identified herein or elsewhere, or to provide an alternative to existing apparatus or methods. In particular it is one object of the invention, amongst others, to provide methods and apparatus for generation of radiation, such as EUV radiation, from a fuel excited to a plasma by an excitation beam, with a high power.